Pain perception is characterized by substantial differences between individuals. We recently showed that a group of experienced yogis tolerated pain much longer than matched controls. To tolerate pain yogis reported using strategies involving interoceptive awareness and parasympathetic activation while nearly all controls did not. Yogis had more gray matter in multiple brain regions, but only mid-insular gray matter correlated with pain tolerance. They also had higher left intra-insular white matter connectivity than controls. Given that autonomic integration occurs in the mid-insula, these observed insular adaptations could be related to successful pain affect regulation mediated by increased interoceptive processing and parasympathetic regulation. Our findings are consistent with a recent theory suggesting that yoga's benefits are achieved by reducing allostatic load in stress response systems, thus restoring optimal homeostasis. Specifically, the beneficial effects of yoga on many conditions, including chronic pain, may be attributable to increased parasympathetic activation. Indeed, all the conditions benefiting from yoga practice are exacerbated by stress and exhibit a high sympathetic/parasympathetic balance, as measured by low heart rate variability (HRV). Increased sympathetic activity is known to increase hypothalamo-pituitary-adrenal (HPA) axis function. Additionally, it has been proposed that people respond to uncertainty by activating the sympathetic nervous system, and that this default response is related to the well-known negativity bias, the tendency to prioritize negative information over positive. Yoga teaches a more neutral appraisal of the world which might be a better adaptive response to uncertainty not involving as much sympathetic activation and requiring less energy from the organism than prioritizing negative information in a reactive way. This should promote a better control over stressful events including pain. Thus, the proposed studies will first pilot the psychophysical procedures before proceeding to the full study testing the general hypothesis that the differences in pain processing and the related neurostructural differences expected to be found in experienced yogis are related to more flexible autonomic and HPA axis control. We will also document personality traits, including pain-related behaviors, and mental states associated with the observed differences.